perf: eliminate Three.js/WebGL, fix render-blocking CSS, add dev team agents

- Replace ShaderGradientBackground WebGL shader (3 static spheres) with pure
  CSS radial-gradient divs — moves from ClientProviders (deferred JS) to
  app/layout.tsx as a server component rendered in initial HTML. Eliminates
  @shadergradient/react, three, @react-three/fiber from the JS bundle.
  Removes chunks/7001 (~20s CPU eval) and the 39s main thread block.

- Remove optimizeCss/critters: it was converting <link rel="stylesheet"> to a
  JS-deferred preload, which PageSpeed read as a 410ms sequential CSS chain.
  Both CSS files now load as parallel <link> tags from initial HTML (~150ms).

- Update browserslist safari >= 15 → 15.4 (Array.prototype.at, Object.hasOwn
  are native in 15.4+; eliminates unnecessary SWC compatibility transforms).

- Delete orphaned app/styles/ghostContent.css (never imported anywhere, 3.7KB).

- Add .claude/ dev team setup: 5 subagents (frontend-dev, backend-dev, tester,
  code-reviewer, debugger), 3 skills (/add-section, /review-changes,
  /check-quality), 3 path-scoped rules, settings.json with auto-lint hook.

- Update CLAUDE.md with server/client orchestrator pattern, SSR animation
  safety rules, API route conventions, and improved command reference.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude/skills/check-quality/SKILL.md

@@ -0,0 +1,39 @@
+---
+name: check-quality
+description: Run all quality checks (lint, build, tests) and report a summary of the project's health
+disable-model-invocation: false
+---
+
+Run all quality checks for this portfolio project and report the results.
+
+Execute these checks in order:
+
+**1. ESLint**
+Run: `npm run lint`
+Required: 0 errors (warnings OK)
+
+**2. TypeScript**
+Run: `npx tsc --noEmit`
+Required: 0 type errors
+
+**3. Unit Tests**
+Run: `npm run test -- --passWithNoTests`
+Report: pass/fail count and any failing test names
+
+**4. Production Build**
+Run: `npm run build`
+Required: successful completion
+
+**5. i18n Parity Check**
+Compare keys in `messages/en.json` vs `messages/de.json` — report any keys present in one but not the other.
+
+After all checks, produce a summary table:
+| Check | Status | Details |
+|-------|--------|---------|
+| ESLint | ✓/✗ | ... |
+| TypeScript | ✓/✗ | ... |
+| Tests | ✓/✗ | X passed, Y failed |
+| Build | ✓/✗ | ... |
+| i18n parity | ✓/✗ | Missing keys: ... |
+
+If anything fails, provide the specific error and a recommended fix.